Methods and compositions for stimulating the production of hydrocarbons from subterranean formations

ABSTRACT

Methods for treating an oil or gas well having a wellbore and selecting compositions for treating an oil or gas well having a wellbore.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/071,332, filed Mar. 16, 2016, which is a continuation of, claimspriority to U.S. application Ser. No. 13/918,155 (now U.S. Pat. No.9,321,955), filed Jun. 14, 2013, which are incorporated herein byreference in their entirety.

FIELD OF INVENTION

The present invention generally provides methods and compositions forstimulating the production of hydrocarbons (e.g., formation crude oiland/or formation gas) from subterranean formations.

BACKGROUND OF INVENTION

For many years, petroleum has been recovered from subterraneanreservoirs through the use of drilled wells and production equipment.During the production of desirable hydrocarbons, such as crude oil andnatural gas, a number of other naturally occurring substances may alsobe encountered within the subterranean environment. The term“stimulation” generally refers to the treatment of geological formationsto improve the recovery of liquid hydrocarbons (e.g., formation crudeoil and/or formation gas). Common stimulation techniques include wellfracturing, slickwater, and acidizing operations.

Oil and natural gas are found in, and produced from, porous andpermeable subterranean formations. The porosity and permeability of theformation determine its ability to store hydrocarbons, and the facilitywith which the hydrocarbons can be extracted from the formation.Hydraulic fracturing is commonly used to stimulate low permeabilitygeological formations to improve the recovery of hydrocarbons. Theprocess can involve suspending chemical agents in a well-treatment fluid(e.g., fracturing fluid) and injecting the fluid down the wellbore.However, the assortment of chemicals pumped down the well can causedamage to the surrounding formation by entering the reservoir rock andblocking the pore throats. It is known that fluid invasion can have adetrimental effect on gas permeability and can impair well productivity.In addition, fluids may become trapped in the formation due to capillaryend effects in and around the vicinity of the formation fractures.

In efforts to reduce phase trapping, additives have been incorporatedinto well-treatment fluids. Generally, the composition of additivescomprises multi-component chemical substances and compositions thatcontain mutually distributed nanodomains of normally immisciblesolvents, such as water and hydrocarbon-based organic solvents,stabilized by surfactants (e.g., microemulsions). The incorporation ofadditives into well-treatment fluids can increase crude oil or formationgas, for example by reducing capillary pressure and/or minimizingcapillary end effects.

Although a number of additives are known in the art, there is acontinued need for more effective additives for increasing crude oil orformation gas for wellbore remediation, drilling operations, andformation stimulation.

SUMMARY OF INVENTION

Methods and compositions for stimulating the production of hydrocarbons(e.g., formation crude oil and/or formation gas) from subterraneanformations are provided.

In some embodiments, a method of selecting a composition for treating anoil or gas well having a wellbore is provided comprising determiningwhether displacement of residual aqueous treatment fluid by formationcrude oil or displacement of residual aqueous treatment fluid byformation gas is preferentially stimulated for the oil or gas wellhaving a wellbore; and selecting an emulsion or a microemulsion forinjection into the wellbore to increase formation crude oil or formationgas production by the well, wherein the emulsion or the microemulsioncomprises water, at least a first type of solvent, and a surfactant,wherein the solvent is selected from the group consisting ofunsubstituted cyclic or acyclic, branched or unbranched alkanes having6-12 carbon atoms, unsubstituted acyclic branched or unbranched alkeneshaving one or two double bonds and 6-12 carbon atoms, cyclic or acyclic,branched or unbranched alkanes having 9-12 carbon atoms and substitutedwith only an —OH group, branched or unbranched dialkylether compoundshaving the formula C_(n)H_(2n+1)OC_(m)H_(2m+1), wherein n+m is between 6and 16, and aromatic solvents having a boiling point between about300-400° F., when displacement of residual aqueous treatment fluid byformation crude oil is preferentially stimulated; or wherein the solventis selected from the group consisting of cyclic or acyclic, branched orunbranched alkanes having 8 carbon atoms and substituted with only an—OH group and aromatic solvents having a boiling point between about175-300° F., when displacement of residual aqueous treatment fluid byformation gas is preferentially stimulated.

In some embodiments, a method of treating an oil or gas well having awellbore is provided comprising injecting an emulsion or a microemulsioninto the wellbore of the oil or gas well to stimulate displacement ofresidual aqueous treatment fluid by formation crude oil and increaseproduction of formation crude oil by the well, wherein the emulsion orthe microemulsion comprises water, at least a first type of solvent, anda surfactant; and wherein the solvent is selected from the groupconsisting of unsubstituted cyclic or acyclic, branched or unbranchedalkanes having 6-12 carbon atoms, unsubstituted acyclic branched orunbranched alkenes having one or two double bonds and 6-12 carbon atoms,cyclic or acyclic, branched or unbranched alkanes having 9-12 carbonatoms and substituted with only an —OH group, branched or unbrancheddialkylether compounds having the formula C_(n)H_(2n+1)OC_(m)H_(2m+1),wherein n+m is between 6 and 16, and aromatic solvents having a boilingpoint between about 300-400° F.

In some embodiments, a method of treating an oil or gas well having awellbore is provided comprising injecting an emulsion or a microemulsioninto the wellbore of the oil or gas well to stimulate displacement ofresidual aqueous treatment fluid by formation gas and increaseproduction of formation gas by the well, wherein the emulsion or themicroemulsion comprises water, at least a first type of solvent, and asurfactant; and wherein the solvent is selected from the groupconsisting of cyclic or acyclic, branched or unbranched alkanes having 8carbon atoms and substituted with only an —OH group and aromaticsolvents having a boiling point between about 175-300° F.

In some embodiments, a composition for injecting into a wellbore isprovided comprising an aqueous carrier fluid and an emulsion or amicroemulsion, wherein the emulsion or the microemulsion is present inan amount between about 0.1 wt % and about 2 wt % versus the totalcomposition, and wherein the emulsion or microemulsion comprises anaqueous phase, a surfactant, a freezing point depression agent, and asolvent comprising an alpha-olefin.

Other aspects, embodiments, and features of the invention will becomeapparent from the following detailed description when considered inconjunction with the accompanying drawings. All patent applications andpatents incorporated herein by reference are incorporated by referencein their entirety. In case of conflict, the present specification,including definitions, will control.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1 shows an exemplary plot for determining the phase inversiontemperature of a microemulsion, according to some embodiments.

DETAILED DESCRIPTION

The present invention generally relates to methods and well-treatmentcompositions (e.g., emulsions or microemulsions) for stimulating of theproduction of hydrocarbons (e.g., formation crude oil and/or formationgas) from subterranean formations. In some embodiments, the compositionscomprise an emulsion or a microemulsion, as described in more detailherein. The emulsions or the microemulsions may include water, asolvent, a surfactant, and optionally a freezing point depression agentor other components. In some embodiments, the solvent comprises morethan one type of solvent (e.g., a first type of solvent and a secondtype of solvent). In some embodiments, the methods relate to stimulatingdisplacement of residual aqueous treatment fluid by formation crude oilor formation gas to increase production of liquid hydrocarbons, asdescribed in more detail below. In some embodiments, methods ofselecting an emulsion or a microemulsion comprising a solvent areprovided, wherein the emulsion or the microemulsion is selected so as toincrease liquid hydrocarbon production. In other embodiments, methods ofselecting an emulsion or a microemulsion comprising a solvent areprovided, wherein the emulsion or the microemulsion is selected so as toincrease gaseous hydrocarbon production. In some embodiments, thesolvent is a hydrocarbon solvent comprising between 6 and 12 carbonatoms. The hydrocarbon may be a linear, branched, or cyclic hydrocarbon,including aromatics, and may be optionally substituted with variousfunctional groups, as described herein.

As described herein, in some embodiments, the inventors have found thatmicroemulsions or emulsions comprising certain solvents increase thedisplacement (e.g., flowback) of residual aqueous treatment fluid byliquid hydrocarbons (e.g., crude oil) as compared to other solvents. Inother embodiments, emulsions or microemulsions comprising certainsolvents increase the displacement of residual aqueous treatment fluidby gaseous hydrocarbons as compared to other solvents. Laboratory testsmay be conducted, as described herein, to determine the displacement ofresidual aqueous treatment fluid by liquid hydrocarbons and/or gaseoushydrocarbons of an emulsion or a microemulsion

Petroleum is generally recovered from subterranean reservoirs throughthe use of drilled wells and production equipment. Wells are“stimulated” using various treatments (e.g., fracturing, acidizing) ofgeological formations to improve the recovery of liquid hydrocarbons.Oil and natural gas are found in, and produced from, porous andpermeable subterranean formations. Based on techniques known in the art,as well as the preference for the desired product isolated (e.g.,formation crude oil or formation gas), it may be preferential tostimulate either crude oil production or gas production from each well.A well drilled into a subterranean formation may penetrate formationscontaining liquid or gaseous hydrocarbons or both, as well as connatewater or brine. The gas-to-oil ratio is termed the GOR. The operator ofthe well may choose to complete the well in such a way as to produce(for example) predominantly liquid hydrocarbons (crude oil).Alternatively, the operator may be fracturing a tight formationcontaining predominantly gaseous hydrocarbons.

Incorporation of the emulsions or the microemulsions described herein(e.g., comprising water, a solvent, and a surfactant) intowell-treatment fluids (e.g., fracturing fluids) can aid in reducingfluid trapping, for example, by reducing capillary pressure and/orminimizing capillary end effects. In addition, incorporation of theemulsions or the microemulsions described herein into well-treatmentfluids can promote increased flowback of aqueous phases following welltreatment, and thus, increase production of liquid and/or gaseoushydrocarbons. That is, incorporation of an emulsion or a microemulsiondescribed herein can aid in the displacement of residual aqueoustreatment fluid by formation crude oil and/or formation gas. Residualaqueous treatment fluids may include those fluids pumped into the well,as well as residual aqueous fluids originally present in the well.

In some embodiments, methods of treating an oil or gas well areprovided. In some embodiments, the methods comprise injecting anemulsion or a microemulsion into the wellbore of the oil or gas well tostimulate displacement of residual aqueous treatment fluid by formationcrude oil or formation gas, and increase production of liquid or gaseoushydrocarbons by the well.

In some embodiments, methods are provided for selecting a compositionfor treating an oil or gas well. The inventors have discovered thatcertain solvents are more effective at stimulating displacement ofresidual aqueous treatment fluid by formation crude oil and others aremore effective for stimulating displacement of residual aqueoustreatment fluid by formation gas for the oil or gas well.

It should be understood, that in embodiments where a microemulsion issaid to be injected into a wellbore, that the microemulsion may bediluted and/or combined with other liquid component(s) prior to and/orduring injection. For example, in some embodiments, the microemulsion isdiluted with an aqueous carrier fluid (e.g., water, brine, sea water,fresh water, or a well-treatment fluid (e.g., an acid, a fracturingfluid comprising polymers, sand, etc., slickwater) prior to and/orduring injection into the wellbore. In some embodiments, a compositionfor injecting into a wellbore is provided comprising a microemulsion asdescribed herein and an aqueous carrier fluid, wherein the microemulsionis present in an amount between about 0.1 and about 50 gallons perthousand gallons of dilution fluid (“gpt”), or between about 0.5 andabout 10 gpt, or between about 0.5 and about 2 gpt.

In some embodiments, emulsions or microemulsion are provided. The termsshould be understood to include emulsions or microemulsions that have awater continuous phase, or that have an oil continuous phase, ormicroemulsions that are bicontinuous.

As used herein, the term “emulsion” is given its ordinary meaning in theart and refers to dispersions of one immiscible liquid in another, inthe form of droplets, with diameters approximately in the range of100-1,000 nanometers. Emulsions may be thermodynamically unstable and/orrequire high shear forces to induce their formation.

As used herein, the term “microemulsion” is given its ordinary meaningin the art and refers to dispersions of one immiscible liquid inanother, in the form of droplets, with diameters approximately in therange of about 10-100 nanometers. Microemulsions are clear ortransparent because they contain particles smaller than the wavelengthof visible light. In addition, microemulsions are homogeneousthermodynamically stable single phases, and form spontaneously, andthus, differ markedly from thermodynamically unstable emulsions, whichgenerally depend upon intense mixing energy for their formation.Microemulsions may be characterized by a variety of advantageousproperties including, by not limited to, (i) clarity, (ii) very smallparticle size, (iii) ultra-low interfacial tensions, (iv) the ability tocombine properties of water and oil in a single homogeneous fluid, (v)shelf stability, and (vi) ease of preparation.

It should be understood, that while much of the description hereinfocuses on microemulsions, this is by no means limiting, and emulsionsmay be employed where appropriate.

In some embodiments, a microemulsion comprises water, a solvent, and asurfactant. In some embodiments, the microemulsion may further compriseadditional components, for example, a freezing point depression agent.Details of each of the components of the microemulsions are described indetail herein. In some embodiments, the components of the microemulsionsare selected so as to reduce or eliminate the hazards of themicroemulsion to the environment and/or the subterranean reservoirs.

The microemulsion generally comprises a solvent. The solvent, or acombination of solvents, may be present in the microemulsion in anysuitable amount. In some embodiments, the total amount of solventpresent in the microemulsion is between about 2 wt % and about 60 wt %,or between about 5 wt % and about 40 wt %, or between about 5 wt % andabout 30 wt %, versus the total microemulsion composition.

The water to solvent ratio in a microemulsion may be varied. In someembodiments, the ratio of water to solvent, along with other parametersof the solvent, may be varied so that displacement of residual aqueoustreatment fluid by formation gas and/or formation crude ispreferentially stimulated. In some embodiments, the ratio of water tosolvent is between about 15:1 and 1:10, or between 9:1 and 1:4, orbetween 3.2:1 and 1:4.

In some embodiments, when displacement of residual aqueous treatmentfluid by formation crude oil is preferentially stimulated, the solventis selected from the group consisting of unsubstituted cyclic oracyclic, branched or unbranched alkanes having 6-12 carbon atoms,unsubstituted acyclic branched or unbranched alkenes having one or twodouble bonds and 6-12 carbon atoms, cyclic or acyclic, branched orunbranched alkanes having 9-12 carbon atoms and substituted with only an—OH group, branched or unbranched dialkylether compounds having theformula C_(n)H_(2n+1)OC_(m)H_(2m+1), wherein n+m is between 6 and 16,and aromatic solvents having a boiling point between about 300-400° F.

In some embodiments, the solvent is an unsubstituted cyclic or acyclic,branched or unbranched alkane having 6-12 carbon atoms. In someembodiments, the cyclic or acyclic, branched or unbranched alkane has6-10 carbon atoms. Non-limiting examples of unsubstituted acyclicunbranched alkanes having 6-12 carbon atoms include hexane, heptane,octane, nonane, decane, undecane, and dodecane. Non-limiting examples ofunsubstituted acyclic branched alkanes having 6-12 carbon atoms includeisomers of methylpentane (e.g., 2-methylpentane, 3-methylpentane),isomers of dimethylbutane (e.g., 2,2-dimethylbutane,2,3-dimethylbutane), isomers of methylhexane (e.g., 2-methylhexane,3-methylhexane), isomers of ethylpentane (e.g., 3-ethylpentane), isomersof dimethylpentane (e.g., 2,2,-dimethylpentane, 2,3-dimethylpentane,2,4-dimethylpentane, 3,3-dimethylpentane), isomers of trimethylbutane(e.g., 2,2,3-trimethylbutane), isomers of methylheptane (e.g.,2-methylheptane, 3-methylheptane, 4-methylheptane), isomers ofdimethylhexane (e.g., 2,2-dimethylhexane, 2,3-dimethylhexane,2,4-dimethylhexane, 2,5-dimethylhexane, 3,3-dimethylhexane,3,4-dimethylhexane), isomers of ethylhexane (e.g., 3-ethylhexane),isomers of trimethylpentane (e.g., 2,2,3-trimethylpentane,2,2,4-trimethylpentane, 2,3,3-trimethylpentane, 2,3,4-trimethylpentane),and isomers of ethylmethylpentane (e.g., 3-ethyl-2-methylpentane,3-ethyl-3-methylpentane). Non-limiting examples of unsubstituted cyclicbranched or unbranched alkanes having 6-12 carbon atoms, includecyclohexane, methylcyclopentane, ethylcyclobutane, propylcyclopropane,isopropylcyclopropane, dimethylcyclobutane, cycloheptane,methylcyclohexane, dimethylcyclopentane, ethylcyclopentane,trimethylcyclobutane, cyclooctane, methylcycloheptane,dimethylcyclohexane, ethylcyclohexane, cyclononane, methylcyclooctane,dimethylcycloheptane, ethylcycloheptane, trimethylcyclohexane,ethylmethylcyclohexane, propylcyclohexane, and cyclodecane. In aparticular embodiment, the unsubstituted cyclic or acyclic, branched orunbranched alkane having 6-12 carbon is selected from the groupconsisting of heptane, octane, nonane, decane, 2,2,4-trimethylpentane(isooctane), and propylcyclohexane.

In some embodiments, the solvent is an unsubstituted acyclic branched orunbranched alkene having one or two double bonds and 6-12 carbon atoms.In some embodiments, the solvent is an unsubstituted acyclic branched orunbranched alkene having one or two double bonds and 6-10 carbon atoms.Non-limiting examples of unsubstituted acyclic unbranched alkenes havingone or two double bonds and 6-12 carbon atoms include isomers of hexene(e.g., 1-hexene, 2-hexene), isomers of hexadiene (e.g., 1,3-hexadiene,1,4-hexadiene), isomers of heptene (e.g., 1-heptene, 2-heptene,3-heptene), isomers of heptadiene (e.g., 1,5-heptadiene, 1-6,heptadiene), isomers of octene (e.g., 1-octene, 2-octene, 3-octene),isomers of octadiene (e.g., 1,7-octadiene), isomers of nonene, isomersof nonadiene, isomers of decene, isomers of decadiene, isomers ofundecene, isomers of undecadiene, isomers of dodecene, and isomers ofdodecadiene. In some embodiments, the acyclic unbranched alkene havingone or two double bonds and 6-12 carbon atoms is an alpha-olefin (e.g.,1-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene,1-dodecene). Non-limiting examples unsubstituted acyclic branchedalkenes include isomers of methylpentene, isomers of dimethylpentene,isomers of ethylpentene, isomers of methylethylpentene, isomers ofpropylpentene, isomers of methylhexene, isomers of ethylhexene, isomersof dimethylhexene, isomers of methylethylhexene, isomers ofmethylheptene, isomers of ethylheptene, isomers of dimethylhexptene, andisomers of methylethylheptene. In a particular embodiment, theunsubstituted acyclic unbranched alkene having one or two double bondsand 6-12 carbon atoms is selected from the group consisting of 1-octeneand 1,7-octadiene.

In some embodiments, the solvent is a cyclic or acyclic, branched orunbranched alkane having 9-12 carbon atoms and substituted with only an—OH group. Non-limiting examples of cyclic or acyclic, branched orunbranched alkanes having 9-12 carbon atoms and substituted with only an—OH group include isomers of nonanol, isomers of decanol, isomers ofundecanol, and isomers of dodecanol. In a particular embodiment, thecyclic or acyclic, branched or unbranched alkane having 9-12 carbonatoms and substituted with only an —OH group is selected from the groupconsisting of 1-nonanol and 1-decanol.

In some embodiments, the solvent is a branched or unbrancheddialkylether compound having the formula C_(n)H_(2n+1)OC_(m)H_(2m+1)wherein n+m is between 6 and 16. In some cases, n+m is between 6 and 12,or between 6 and 10, or between 6 and 8. Non-limiting examples ofbranched or unbranched dialkylether compounds having the formulaC_(n)H_(2n+1)OC_(m)H_(2m+1) include isomers of C₃H₇OC₃H₇, isomers ofC₄H₉OC₃H₇, isomers of C₅H₁₁OC₃H₇, isomers of C₆H₁₃OC₃H₇, isomers ofC₄H₉OC₄H₉, isomers of C₄H₉OC₅H₁₁, isomers of C₄H₉OC₆H₁₃, isomers ofC₅H₁₁OC₆H₁₃, and isomers of C₆H₁₃OC₆H₁₃. In a particular embodiment, thebranched or unbranched dialklyether is an isomer C₆H₁₃OC₆H₁₃ (e.g.,dihexylether).

In some embodiments, the solvent is an aromatic solvent having a boilingpoint between about 300-400° F. Non-limiting examples of aromaticsolvents having a boiling point between about 300-400° F. includebutylbenzene, hexylbenzene, mesitylene, light aromatic naphtha, andheavy aromatic naphtha.

In other embodiments, when displacement of residual aqueous treatmentfluid by formation gas is preferentially stimulated, the solvent isselected from the group consisting of cyclic or acyclic, branched orunbranched alkanes having 8 carbon atoms and substituted only with an—OH group and aromatic solvents having a boiling point between about175-300° F.

In some embodiments, the solvent is a cyclic or acyclic, branched orunbranched alkane having 8 carbon atoms and substituted with only an —OHgroup. Non-limiting examples of cyclic or acyclic, branched orunbranched alkanes having 8 carbon atoms and substituted with only an—OH group include isomers of octanol (e.g., 1-octanol, 2-octanol,3-octanol, 4-octanol), isomers of methyl heptanol, isomers ofethylhexanol (e.g., 2-ethyl-1-hexanol, 3-ethyl-1-hexanol,4-ethyl-1-hexanol), isomers of dimethylhexanol, isomers ofpropylpentanol, isomers of methylethylpentanol, and isomers oftrimethylpentanol. In a particular embodiment, the cyclic or acyclic,branched or unbranched alkane having 8 carbon atoms and substituted withonly an —OH group is selected from the group consisting of 1-octanol and2-ethyl-1-hexanol.

In some embodiments, the solvent is an aromatic solvent having a boilingpoint between about 175-300° F. Non-limiting examples of aromatic liquidsolvents having a boiling point between about 175-300° F. includebenzene, xylenes, and toluene. In a particular embodiment, the solventis not xylene.

In some embodiments, the microemulsion comprises a first type of solventand a second type of solvent. The first type of solvent to the secondtype of solvent ratio in a microemulsion may be present in any suitableratio. In some embodiments, the ratio of the first type of solvent tothe second type of solvent is between about 4:1 and 1:4, or between 2:1and 1:2, or about 1:1.

In some cases, when displacement of residual aqueous treatment fluid byformation crude oil is preferentially stimulated, the first type ofsolvent and the second type of solvent are different and are selectedfrom the group consisting of unsubstituted cyclic or acyclic, branchedor unbranched alkanes having 6-12 carbon atoms, unsubstituted acyclicbranched or unbranched alkenes having one or two double bonds and 6-12carbon atoms, cyclic or acyclic, branched or unbranched alkanes having9-12 carbon atoms and substituted with only an —OH group, branched orunbranched dialkylether compounds having the formulaC_(n)H_(2n+1)OC_(m)H_(2m+1), wherein n+m is between 6 and 16, andaromatic solvents having a boiling point between about 300-400° F. Inother embodiments, when displacement of residual aqueous treatment fluidby formation gas is preferentially stimulated, the first type of solventand the second type of solvent are different and are selected from thegroup consisting of cyclic or acyclic, branched or unbranched alkaneshaving 8 carbon atoms and substituted with only an —OH group andaromatic solvents having a boiling point between about 175-300° F.

In some embodiments, at least one solvent present in the microemulsionis a terpene or terpenoid. In some cases, when displacement of residualaqueous treatment fluid by formation crude oil is preferentiallystimulated, the first type of solvent is selected from the groupconsisting of unsubstituted cyclic or acyclic, branched or unbranchedalkanes having 6-12 carbon atoms, unsubstituted acyclic branched orunbranched alkenes having one or two double bonds and 6-12 carbon atoms,cyclic or acyclic, branched or unbranched alkanes having 9-12 carbonatoms and substituted with only an —OH group, branched or unbrancheddialkylether compounds having the formula C_(n)H_(2n+1)OC_(m)H_(2m+1),wherein n+m is between 6 and 16, and aromatic solvents having a boilingpoint between about 300-400° F. and the second type of solvent is aterpene or terpenoid. In some cases, the terpene or terpenoid may beselected so as to preferentially stimulate displacement of residualaqueous treatment fluid by formation crude oil. In such embodiments, theterpene or terpenoid for preferentially stimulating displacement ofresidual aqueous treatment fluid by formation crude oil may have a phaseinversion temperature greater than 109.4° F., as determined by themethod described herein.

In other embodiments, when displacement of residual aqueous treatmentfluid by formation gas is preferentially stimulated, the first type ofsolvent is selected from the group consisting of cyclic or acyclic,branched or unbranched alkanes having 8 carbon atoms and substitutedwith only an —OH group and aromatic solvents having a boiling pointbetween about 175-300° F. and the second type of solvent is a terpene orterpenoid. In some cases, the terpene or terpenoid may be selected so asto preferentially stimulate displacement of residual aqueous treatmentfluid by formation gas. In such embodiments, the terpene or terpenoidfor preferentially stimulating displacement of residual aqueoustreatment fluid by formation gas may have a phase inversion temperatureless than 109.4° F., as determined by the method described herein.

Those of ordinary skill in the art will appreciate that microemulsionscomprising more than two types of solvents may be utilized in themethods, compositions, and systems described herein. For example, themicroemulsion may comprise more than one or two types of solvent, forexample, three, four, five, six, or more, types of solvents. As anon-limiting example, when displacement of residual aqueous treatmentfluid by formation crude oil is preferentially stimulated, themicroemulsion may comprise one or more solvents selected from the groupconsisting of unsubstituted cyclic or acyclic, branched or unbranchedalkanes having 6-12 carbon atoms, unsubstituted acyclic branched orunbranched alkenes having one or two double bonds and 6-12 carbon atoms,cyclic or acyclic, branched or unbranched alkanes having 9-12 carbonatoms and substituted with only an —OH group, branched or unbrancheddialkylether compounds having the formula C_(n)H_(2n+1)OC_(m)H_(2m+1),wherein n+m is between 6 and 16, and aromatic solvents having a boilingpoint between about 300-400° F. and one or more terpenes or terpenoids.As another non-limiting example, when displacement of residual aqueoustreatment fluid by formation gas is preferentially stimulated, themicroemulsion may comprise one or more solvents selected from the groupconsisting of cyclic or acyclic, branched or unbranched alkanes having 8carbon atoms and substituted with only an —OH group and aromaticsolvents having a boiling point between about 175-300° F. and one ormore terpenes or terpenoids.

In some embodiments, at least one of the solvents present in themicroemulsion is a terpene or a terpenoid. In some embodiments, theterpene or terpenoid comprises a first type of terpene or terpenoid anda second type of terpene or terpenoid. Terpenes may be generallyclassified as monoterpenes (e.g., having two isoprene units),sesquiterpenes (e.g., having 3 isoprene units), diterpenes, or the like.The term terpenoid also includes natural degradation products, such asionones, and natural and synthetic derivatives, e.g., terpene alcohols,aldehydes, ketones, acids, esters, epoxides, and hydrogenation products(e.g., see Ullmann's Encyclopedia of Industrial Chemistry, 2012, pages29-45, herein incorporated by reference). It should be understood, thatwhile much of the description herein focuses on terpenes, this is by nomeans limiting, and terpenoids may be employed where appropriate.

In some embodiments, the terpene is a monoterpene. Monoterpenes may befurther classified as acyclic, monocyclic, and bicyclic [18-20], as wellas whether the monoterpene comprises one or more oxygen atoms (e.g.,alcohol groups, ester groups, carbonyl groups, etc.). In someembodiments, the terpene comprises an alcohol group. Non-limitingexamples of terpenes comprising an alcohol group are linalool, geraniol,nopol, α-terpineol, and menthol. In some embodiments, the terpenecomprises an ether-oxygen, for example, eucalyptol, or a carbonyloxygen, for example, menthone. In some embodiments, the terpene does notcomprise an oxygen atom, for example, d-limonene.

Non-limiting examples of terpenes include linalool, geraniol, nopol,α-terpineol, menthol, eucalyptol, menthone, d-limonene, terpinolene,β-occimene, γ-terpinene, α-pinene, and citronellene. In a particularembodiment, the terpene is selected from the group consisting ofα-terpeneol, α-pinene, nopol, and eucalyptol. In one embodiment, theterpene is nopol. In another embodiment, the terpene is eucalyptol. Insome embodiments, the terpene is not limonene (e.g., d-limonene). Insome embodiments, the emulsion is free of limonene

In some embodiments, the terpene may be classified in terms of its phaseinversion temperature (“PIT”). The term “phase inversion temperature” isgiven its ordinary meaning in the art and refers to the temperature atwhich an oil in water microemulsion inverts to a water in oilmicroemulsion (or vice versa). Those of ordinary skill in the art willbe aware of methods for determining the PIT for a microemulsioncomprising a terpene (e.g., see Strey, Colloid & Polymer Science, 1994.272(8): p. 1005-1019; Kahlweit et al., Angewandte Chemie InternationalEdition in English, 1985. 24(8): p. 654-668). The PIT values describedherein were determined using a 1:1 ratio of terpene (e.g., one or moreterpenes):de-ionized water and varying amounts (e.g., between about 20wt % and about 60 wt %; generally, between 3 and 9 different amounts areemployed) of a 1:1 blend of surfactant comprising linear C₁₂-C₁₅ alcoholethoxylates with on average 7 moles of ethylene oxide (e.g., Neodol25-7):isopropyl alcohol wherein the upper and lower temperatureboundaries of the microemulsion region can be determined and a phasediagram may be generated. Those of ordinary skill in the art willrecognize that such a phase diagram (e.g., a plot of temperature againstsurfactant concentration at a constant oil-to-water ratio) may bereferred to as “fish” diagram or a Kahlweit plot. The temperature at thevertex is the PIT. An exemplary fish diagram indicating the PIT is shownin FIG. 1. PITs for non-limiting examples of terpenes determined usingthis experimental procedure outlined above are given in Table 1.

TABLE 1 Phase inversion temperatures for non- limiting examples ofterpenes. Phase Inversion Terpene Temperature ° F. (° C.) linalool 24.8(−4) geraniol 31.1 (−0.5) nopol 36.5 (2.5) α-terpineol 40.3 (4.6)menthol 60.8 (16) eucalyptol 87.8 (31) menthone 89.6 (32) d-limonene109.4 (43) terpinolene 118.4 (48) β-occimene 120.2 (49) γ-terpinene120.2 (49) α-pinene 134.6 (57) citronellene 136.4 (58)

In some embodiments, the terpene has a PIT greater than and/or less than109.4° F., as determined by the method described herein. In someembodiments, the terpene has a PIT greater than 109.4° F., as determinedby the method described herein. In some embodiments, the terpene has aPIT less than 109.4° F., as determined by the method described herein.In some embodiments, the terpene has a PIT greater than 89.6° F., asdetermined by the method described herein. In some embodiments, theterpene has a PIT less than 89.6 F as determined by the method describedherein. In some embodiments, the PIT is between about 14° F. and about158° F., or between about 24.8° F. and about 140° F., as determined bythe method described herein. In some embodiments, the minimum PIT is 14°F., or 24.8° F., as determined by the method described herein. In someembodiments, the maximum PIT is 158° F., or 140° F., as determined bythe method described herein.

In some embodiments, if displacement of residual aqueous treatment fluidby formation crude oil is preferentially stimulated and the emulsion orthe microemulsion comprises water, a first type of solvent (e.g., asdescribed above), and a terpene, then the terpene may be selected tohave a phase inversion temperature greater than 109.4° F., as determinedby the method described herein. Alternatively, if displacement ofresidual aqueous treatment fluid by formation gas is preferentiallystimulated and the emulsion or the microemulsion comprises water, afirst type of solvent (e.g., as described herein), and a terpene, thenthe terpene may be selected to have a phase inversion temperature lessthan 109.4° F., as determined by the method described herein

In some embodiments, the microemulsion comprises a surfactant. Themicroemulsion may comprise a single surfactant or a combination of twoor more surfactants. For example, in some embodiments, the surfactantcomprises a first type of surfactant and a second type of surfactant.The term “surfactant,” as used herein, is given its ordinary meaning inthe art and refers to compounds having an amphiphilic structure whichgives them a specific affinity for oil/water-type and water/oil-typeinterfaces which helps the compounds to reduce the free energy of theseinterfaces and to stabilize the dispersed phase of a microemulsion. Theterm surfactant encompasses cationic surfactants, anionic surfactants,amphoteric surfactants, nonionic surfactants, zwitterionic surfactants,and mixtures thereof. In some embodiments, the surfactant is a nonionicsurfactant. Nonionic surfactants generally do not contain any charges.Amphoteric surfactants generally have both positive and negativecharges, however, the net charge of the surfactant can be positive,negative, or neutral, depending on the pH of the solution. Anionicsurfactants generally possess a net negative charge. Cationicsurfactants generally possess a net positive charge.

Suitable surfactants for use with the compositions and methods describedherein will be known in the art. In some embodiments, the surfactant isan alkyl polyglycol ether, for example, having 2-40 ethylene oxide (EO)units and alkyl groups of 4-20 carbon atoms. In some embodiments, thesurfactant is an alkylaryl polyglycol ether having 2-40 EO units and8-20 carbon atoms in the alkyl and aryl groups. In some embodiments, thesurfactant is an ethylene oxide/propylene oxide (EO/PO) block copolymerhaving 8-40 EO or PO units. In some embodiments, the surfactant is afatty acid polyglycol ester having 6-24 carbon atoms and 2-40 EO units.In some embodiments, the surfactant is a polyglycol ether ofhydroxyl-containing triglycerides (e.g., castor oil). In someembodiments, the surfactant is an alkylpolyglycoside of the generalformula R″—O—Z_(n), where R″ denotes a linear or branched, saturated orunsaturated alkyl group having on average 8-24 carbon atoms and Z_(n)denotes an oligoglycoside group having on average n=1-10 hexose orpentose units or mixtures thereof. In some embodiments, the surfactantis a fatty ester of glycerol, sorbitol, or pentaerythritol. In someembodiments, the surfactant is an amine oxide (e.g.,dodecyldimethylamine oxide). In some embodiments, the surfactant is analkyl sulfate, for example having a chain length of 8-18 carbon atoms,alkyl ether sulfates having 8-18 carbon atoms in the hydrophobic groupand 1-40 ethylene oxide (EO) or propylene oxide (PO) units. In someembodiments, the surfactant is a sulfonate, for example, an alkylsulfonate having 8-18 carbon atoms, an alkylaryl sulfonate having 8-18carbon atoms, an ester or half ester of sulfosuccinic acid withmonohydric alcohols or alkylphenols having 4-15 carbon atoms. In somecases, the alcohol or alkylphenol can also be ethoxylated with 1-40 EOunits. In some embodiments, the surfactant is an alkali metal salt orammonium salt of a carboxylic acid or poly(alkylene glycol) ethercarboxylic acid having 8-20 carbon atoms in the alkyl, aryl, alkaryl oraralkyl group and 1-40 EO or PO units. In some embodiments, thesurfactant is a partial phosphoric ester or the corresponding alkalimetal salt or ammonium salt, e.g. an alkyl and alkaryl phosphate having8-20 carbon atoms in the organic group, an alkylether phosphate oralkarylether phosphate having 8-20 carbon atoms in the alkyl or alkarylgroup and 1-40 EO units. In some embodiments, the surfactant is a saltof primary, secondary, or tertiary fatty amine having 8-24 carbon atomswith acetic acid, sulfuric acid, hydrochloric acid, and phosphoric acid.In some embodiments, the surfactant is a quaternary alkyl- andalkylbenzylammonium salt, whose alkyl groups have 1-24 carbon atoms(e.g., a halide, sulfate, phosphate, acetate, or hydroxide salt). Insome embodiments, the surfactant is an alkylpyridinium, analkylimidazolinium, or an alkyloxazolinium salt whose alkyl chain has upto 18 carbons atoms (e.g., a halide, sulfate, phosphate, acetate, orhydroxide salt). In some embodiments, the surfactant is amphoteric,including sultaines (e.g., cocamidopropyl hydroxysultaine), betaines(e.g., cocamidopropyl betaine), or phosphates (e.g., lecithin).Non-limiting examples of specific surfactants include a linear C₁₂-C₁₅ethoxylated alcohols with 5-12 moles of EO, lauryl alcohol ethoxylatewith 4-8 moles of EO, nonyl phenol ethoxylate with 5-9 moles of EO,octyl phenol ethoxylate with 5-9 moles of EO, tridecyl alcoholethoxylate with 5-9 moles of EO, Pluronic® matrix of EO/PO copolymers,ethoxylated cocoamide with 4-8 moles of EO, ethoxylated coco fatty acidwith 7-11 moles of EO, and cocoamidopropyl amine oxide.

Those of ordinary skill in the art will be aware of methods andtechniques for selecting surfactants for use in the microemulsionsdescribed herein. In some cases, the surfactant(s) are matched to and/oroptimized for the particular oil or solvent in use. In some embodiments,the surfactant(s) are selected by mapping the phase behavior of themicroemulsion and choosing the surfactant(s) that gives the desiredrange of stability. In some cases, the stability of the microemulsionover a wide range of temperatures is targeted as the microemulsion maybe subject to a wide range of temperatures due to the environmentalconditions present at the subterranean formation.

The surfactant may be present in the microemulsion in any suitableamount. In some embodiments, the surfactant is present in an amountbetween about 10 wt % and about 60 wt %, or between about 15 wt % andabout 55 wt % versus the total microemulsion composition, or betweenabout 20 wt % and about 50 wt %, versus the total microemulsioncomposition.

In some embodiments, the microemulsion comprises a freezing pointdepression agent. The microemulsion may comprise a single freezing pointdepression agent or a combination of two or more freezing pointdepression agents. For example, in some embodiments, the freezing pointdepression agent comprises a first type of freezing point depressionagent and a second type of freezing point depression agent. The term“freezing point depression agent” is given its ordinary meaning in theart and refers to a compound which is added to a solution to reduce thefreezing point of the solution. That is, a solution comprising thefreezing point depression agent has a lower freezing point as comparedto an essentially identical solution not comprising the freezing pointdepression agent. Those of ordinary skill in the art will be aware ofsuitable freezing point depression agents for use in the microemulsionsdescribed herein. Non-limiting examples of freezing point depressionagents include primary, secondary, and tertiary alcohols with between 1and 20 carbon atoms. In some embodiments, the alcohol comprises at least2 carbon atoms, alkylene glycols including polyalkylene glycols, andsalts. Non-limiting examples of alcohols include methanol, ethanol,i-propanol, n-propanol, t-butanol, n-butanol, n-pentanol, n-hexanol, and2-ethyl-hexanol. In some embodiments, the freezing point depressionagent is not methanol (e.g., due to toxicity). Non-limiting examples ofalkylene glycols include ethylene glycol (EG), polyethylene glycol(PEG), propylene glycol (PG), and triethylene glycol (TEG). In someembodiments, the freezing point depression agent is not ethylene oxide(e.g., due to toxicity). Non-limiting examples of salts include saltscomprising K, Na, Br, Cr, Cr, Cs, or Bi, for example, halides of thesemetals, including NaCl, KCl, CaCl₂, and MgCl. In some embodiments, thefreezing point depression agent comprises an alcohol and an alkyleneglycol. Another non-limiting example of a freezing point depressionagent is a combination of choline chloride and urea. In someembodiments, the microemulsion comprising the freezing point depressionagent is stable over a wide range of temperatures, for example, betweenabout −25° F. to 150° F.

The freezing point depression agent may be present in the microemulsionin any suitable amount. In some embodiments, the freezing pointdepression agent is present in an amount between about 1 wt % and about40 wt %, or between about 3 wt % and about 20 wt %, or between about 8wt % and about 16 wt %, versus the total microemulsion composition.

In some embodiments, the components of the microemulsion and/or theamounts of the components may be selected so that the microemulsion isstable over a wide-range of temperatures. For example, the microemulsionmay exhibit stability between about −40° F. and about 300° F., orbetween about −40° F. and about 150° F. Those of ordinary skill in theart will be aware of methods and techniques for determining the range ofstability of the microemulsion. For example, the lower boundary may bedetermined by the freezing point and the upper boundary may bedetermined by the cloud point and/or using spectroscopy methods.Stability over a wide range of temperatures may be important inembodiments where the microemulsions are being employed in applicationscomprising environments wherein the temperature may vary significantly,or may have extreme highs (e.g., desert) or lows (e.g., artic).

In some embodiments, emulsions or microemulsions are provided comprisingwater, a solvent, and a surfactant, wherein the solvents and surfactantsmay be as described herein. In some embodiments, as described herein,the solvent comprises more than one type of solvent, for example, two,three, four, five, six, or more, types of solvents. In some embodiment,at least one solvent is selected from the group consisting ofunsubstituted cyclic or acyclic, branched or unbranched alkanes having6-12 carbon atoms, unsubstituted acyclic branched or unbranched alkeneshaving one or two double bonds and 6-12 carbon atoms, cyclic or acyclic,branched or unbranched alkanes having 9-12 carbon atoms and substitutedwith only an —OH group, branched or unbranched dialkylether compoundshaving the formula C_(n)H_(2n+1)OC_(m)H_(2m+1), wherein n+m is between 6and 16, and aromatic solvents having a boiling point between about300-400° F. In another embodiment, at least one solvent is selected fromthe group consisting of cyclic or acyclic, branched or unbranchedalkanes having 8 carbon atoms and substituted with only an —OH group andaromatic solvents having a boiling point between about 175-300° F. Insome cases, at least one solvent is a terpene. The microemulsion mayfurther comprise addition components, for example, a freezing pointdepression agent. In some embodiments, at least one solvent is selectedfrom the group consisting of butylbenzene, heavy aromatic naphtha, lightaromatic naphtha, 1-nonanol, propylcyclohexane, 1-decanol, dihexylether,1,7-octadiene, hexylbenzene, nonane, decane, 1-octene, isooctane,octane, heptane, mesitylene, xylenes, toluene, 2-ethyl-1-hexanol,1-octanol. In some embodiments, at least one solvent is selected fromthe group consisting of butylbenzene, heavy aromatic naphtha, lightaromatic naphtha, 1-nonanol, propylcyclohexane, 1-decanol, dihexylether,1,7-octadiene, hexylbenzene, nonane, decane, 1-octene, isooctane,octane, heptane, mesitylene, toluene, 2-ethyl-1-hexanol, 1-octanol. Insome embodiments, the at least one solvent is not xylene. In someembodiment, at least one solvent is an alpha-olefin.

In some embodiments, composition for injecting into a wellbore areprovided comprising an aqueous carrier fluid, and an emulsion or amicroemulsion as described herein, wherein the emulsion or themicroemulsion is present in an amount between about 0.1 wt % and about 2wt % versus the total composition. In some embodiments, the emulsion ormicroemulsion comprises an aqueous phase, a surfactant, a freezing pointdepression agent, and a solvent as described herein. In someembodiments, the solvent is as described herein. In some cases, thesolvent comprises an alpha-olefin, for example, having between 6-12carbon atoms. In other cases, the solvent comprises a cyclic or acyclic,branched or unbranched alkane having 8-12, or 9-12, or 8, or 9, or 10,or 11, or 12 carbon atoms and substituted with only an —OH group. Insome cases, the total amount of solvent present in the emulsion ormicroemulsion is between about 2 wt % and about 60 wt % and/or the ratioof the aqueous phase to solvent in the emulsion or microemulsion isbetween 15:1 and 1:10. In some cases, the composition may comprise morethan one type of solvent. In some cases, the solvent comprises analpha-olefin and a terpene. In some cases, the solvent comprises acyclic or acyclic, branched or unbranched alkane having 8-12 carbonatoms and substituted with only an —OH group and a terpene.

The microemulsions described herein may be formed using methods known tothose of ordinary skill in the art. In some embodiments, the aqueous andnon-aqueous phases may be combined (e.g., the water and the solvent(s)),followed by addition of a surfactant(s) and optionally other components(e.g., freezing point depression agent(s)) and agitation. The strength,type, and length of the agitation may be varied as known in the artdepending on various factors including the components of themicroemulsion, the quantity of the microemulsion, and the resulting typeof microemulsion formed. For example, for small samples, a few secondsof gentle mixing can yield a microemulsion, whereas for larger samples,longer agitation times and/or stronger agitation may be required.Agitation may be provided by any suitable source, for example, a vortexmixer, a stirrer (e.g., magnetic stirrer), etc.

Any suitable method for injecting the microemulsion (e.g., a dilutedmicroemulsion) into a wellbore may be employed. For example, in someembodiments, the microemulsion, optionally diluted, may be injected intoa subterranean formation by injecting it into a well or wellbore in thezone of interest of the formation and thereafter pressurizing it intothe formation for the selected distance. Methods for achieving theplacement of a selected quantity of a mixture in a subterraneanformation are known in the art. The well may be treated with themicroemulsion for a suitable period of time. The microemulsion and/orother fluids may be removed from the well using known techniques,including producing the well.

In some embodiments, experiments may be carried out to determinedisplacement of residual aqueous treatment fluid by formation crude oilor formation gas by a microemulsion (e.g., a diluted microemulsion). Forexample, displacement of residual aqueous treatment fluid by formationcrude oil may be determined using the method described in Example 2and/or displacement of residual aqueous treatment fluid by formation gasmay be determined using the method described in Example 3.

For convenience, certain terms employed in the specification, examples,and appended claims are listed here.

Definitions of specific functional groups and chemical terms aredescribed in more detail below. For purposes of this invention, thechemical elements are identified in accordance with the Periodic Tableof the Elements, CAS version, Handbook of Chemistry and Physics, 75^(th)Ed., inside cover, and specific functional groups are generally definedas described therein. Additionally, general principles of organicchemistry, as well as specific functional moieties and reactivity, aredescribed in Organic Chemistry, Thomas Sorrell, University ScienceBooks, Sausalito: 1999, the entire contents of which are incorporatedherein by reference.

Certain compounds of the present invention may exist in particulargeometric or stereoisomeric forms. The present invention contemplatesall such compounds, including cis- and trans-isomers, R- andS-enantiomers, diastereomers, (D)-isomers, (L)-isomers, the racemicmixtures thereof, and other mixtures thereof, as falling within thescope of the invention. Additional asymmetric carbon atoms may bepresent in a substituent such as an alkyl group. All such isomers, aswell as mixtures thereof, are intended to be included in this invention.

Isomeric mixtures containing any of a variety of isomer ratios may beutilized in accordance with the present invention. For example, whereonly two isomers are combined, mixtures containing 50:50, 60:40, 70:30,80:20, 90:10, 95:5, 96:4, 97:3, 98:2, 99:1, or 100:0 isomer ratios areall contemplated by the present invention. Those of ordinary skill inthe art will readily appreciate that analogous ratios are contemplatedfor more complex isomer mixtures.

The term “aliphatic,” as used herein, includes both saturated andunsaturated, nonaromatic, straight chain (i.e., unbranched), branched,acyclic, and cyclic (i.e., carbocyclic) hydrocarbons, which areoptionally substituted with one or more functional groups. As will beappreciated by one of ordinary skill in the art, “aliphatic” is intendedherein to include, but is not limited to, alkyl, alkenyl, alkynyl,cycloalkyl, cycloalkenyl, and cycloalkynyl moieties. Thus, as usedherein, the term “alkyl” includes straight, branched and cyclic alkylgroups. An analogous convention applies to other generic terms such as“alkenyl”, “alkynyl”, and the like. Furthermore, as used herein, theterms “alkyl”, “alkenyl”, “alkynyl”, and the like encompass bothsubstituted and unsubstituted groups. In certain embodiments, as usedherein, “aliphatic” is used to indicate those aliphatic groups (cyclic,acyclic, substituted, unsubstituted, branched or unbranched) having 1-20carbon atoms. Aliphatic group substituents include, but are not limitedto, any of the substituents described herein, that result in theformation of a stable moiety (e.g., aliphatic, alkyl, alkenyl, alkynyl,heteroaliphatic, heterocyclic, aryl, heteroaryl, acyl, oxo, imino,thiooxo, cyano, isocyano, amino, azido, nitro, hydroxyl, thiol, halo,aliphaticamino, heteroaliphaticamino, alkylamino, heteroalkylamino,arylamino, heteroarylamino, alkylaryl, arylalkyl, aliphaticoxy,heteroaliphaticoxy, alkyloxy, heteroalkyloxy, aryloxy, heteroaryloxy,aliphaticthioxy, heteroaliphaticthioxy, alkylthioxy, heteroalkylthioxy,arylthioxy, heteroarylthioxy, acyloxy, and the like, each of which mayor may not be further substituted).

The term “alkane” is given its ordinary meaning in the art and refers toa saturated hydrocarbon molecule. The term “branched alkane” refers toan alkane that includes one or more branches, while the term “unbranchedalkane” refers to an alkane that is straight-chained. The term “cyclicalkane” refers to an alkane that includes one or more ring structures,and may be optionally branched. The term “acyclic alkane” refers to analkane that does not include any ring structures, and may be optionallybranched.

The term “alkene” is given its ordinary meaning in the art and refers toan unsaturated hydrocarbon molecule that includes one or morecarbon-carbon double bonds. The term “branched alkene” refers to analkene that includes one or more branches, while the term “unbranchedalkene” refers to an alkene that is straight-chained. The term “cyclicalkene” refers to an alkene that includes one or more ring structures,and may be optionally branched. The term “acyclic alkene” refers to analkene that does not include any ring structures, and may be optionallybranched.

The term “aromatic” is given its ordinary meaning in the art and refersto aromatic carbocyclic groups, having a single ring (e.g., phenyl),multiple rings (e.g., biphenyl), or multiple fused rings in which atleast one is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl,anthryl, or phenanthryl). That is, at least one ring may have aconjugated pi electron system, while other, adjoining rings can becycloalkyls, cycloalkenyls, cycloalkynyls, aryls and/or heterocyclyls.

These and other aspects of the present invention will be furtherappreciated upon consideration of the following Examples, which areintended to illustrate certain particular embodiments of the inventionbut are not intended to limit its scope, as defined by the claims.

EXAMPLES Example 1

A series of laboratory tests were conducted to characterize theeffectiveness of a series of microemulsions incorporating a variety ofsolvents. For these experiments, samples of a base microemulsion wereprepared in which a detergent range alcohol ethoxylate surfactant wasfirst blended in a 1:1 ratio with isopropyl alcohol. The surfactantemployed in the tests was Neodol 25-7 (obtained from Shell Chemical Co.;e.g., a surfactant comprising linear C₁₂-C₁₅ alcohol ethoxylates with onaverage 7 moles of ethylene oxide). The microemulsions in Table 2 wereprepared by mixing 46 parts by weight of this blend with 27 parts byweight a solvent as shown in Table 2 and 27 parts by weight of water,with the exception of nonane and decane, which were prepared by mixing50 parts of the blend with 25 parts by weight of solvent and 25 partsper weight of water. Two emulsions were also prepared using the samemethod comprising a mixture of a hydrocarbon solvent and a terpene (1:1ratio of hydrocarbon:terpene). The solvents were obtained throughcommercial sources. The heavy aromatic naphtha employed was ExxonAromatic 150 Fluid which comprises C₁₀-12 alkyl benzenes and has adistillation temperature between 363-396° F. and the light aromaticnaphtha employed was Exxon Aromatic 100 Fluid which comprises C₉₋₁₀dialkyl and trialkylbenzenes and has a distillation temperature between322-340° F.

The mixtures were identified as a microemulsion based on the spontaneousformation with minimal mechanical energy input to form a cleardispersion from an immiscible mixture of water and solvent upon additionof an appropriate amount of surfactant. The order of mixing of this andother compositions described in this example were not necessary, but forconvenience, a procedure was generally followed in which a mixture ofthe surfactant and the isopropyl alcohol was first prepared thencombined that with a mixture of the solvent and water. With smallsamples, in the laboratory, a few seconds of gentle mixing yielded atransparent dispersion.

Subsequently, 2 gallons per thousand (gpt) dilutions of themicroemulsions were prepared and tested. The dilutions comprise 0.2 wt %of the microemulsion in 2 wt % KCl solution. The process employeddispensing 200 microliters of the microemulsion into a vortex of avigorously stirred beaker containing 100 mL of 2 wt % KCl, generally atroom temperature (e.g., about 25° C.).

TABLE 2 Maximum Displacement Displacement by Oil at 60 Boiling CAS #Solvent by Gas (%) min (%) point (° F.) 104-51-8 butylbenzene 30.0 89.4361.9 64742-94-5 heavy aromatic naphtha 33.6 91.5 363-396 64742-95-6light aromatic naphtha 36.4 53.7 322-340 143-08-8 1-nonanol 41.2 92.91678-92-8 propylcyclohexane 45.6 91.1 112-30-1 1-decanol 47.5 93.0112-58-3 dihexylether 50.1 92.5 3710-30-3 1,7-octadiene 50.3 92.31077-16-3 hexylbenzene 55.3 90.5 439 111-84-2 nonane 55.9 90.1 124-18-5decane 56.5 82.5 111-66-0 1-octene 56.9 90.8 540-84-1 isooctane 58.884.4 111-65-9 octane 60.8 89.1 540-84-1 heptane 63.0 89.5 108-67-8mesitylene 33.2 80.2 328.5 1330-20-7 xylenes 66.6 54.1 281.3 108-88-3toluene 65.7 232 104-76-7 2-ethyl-1-hexanol 60.5 35.5 111-87-5 1-octanol84.8 67.4 1:1 d-limonene 49.6 88.7 and octane 1:1 alpha-terpineol 86.249.5 and octanol

Tables 3 and 4 provide data related to microemulsions comprising octanewherein the water to oil ratio and the surfactant were varied. Thecomponents of the formulation are given in Table 4 and the results areprovided in Table 3. The greater efficacy of displacement of residualaqueous treatment fluid for the microemulsions comprising octane bycrude oil compared with gaseous hydrocarbon was maintained over therange of water to oil ratio of 3.2:1 to 1:4 or surfactant/co-solventconcentrations from 40-50.

TABLE 3 Effectiveness of brine displacement by gas and oil using amicroemulsion comprising octane. Maximum Displacement of Experiment #displacementof brine (%) by crude (water-to-oil ratio) brine (%) by gasoil at 120 minutes 1 (3.2:1) 60 91 2 (1.8:1) 65 92 3 (9:1) 57 93 4 (1:4)50 92 5 (9:1) 55 92

TABLE 4 Formulation compositions 1:1 Blend of Neodol 25-7 Experiment DIwater Octane and IPA # (wt %) (wt %) (wt %) 1 38 12 50 2 32 18 50 3 45 550 4 10 40 50 5 54 6 40

Example 2

This example described a non-limiting experiment for determiningdisplacement of residual aqueous treatment fluid by formation crude oil.A 25 cm long, 2.5 cm diameter capped glass chromatography column waspacked with 77 grams of 100 mesh sand. The column was left open on oneend and a PTFE insert containing a recessed bottom, 3.2 mm diameteroutlet, and nipple was placed into the other end. Prior to placing theinsert into the column, a 3 cm diameter filter paper disc (Whatman, #40)was pressed firmly into the recessed bottom of the insert to preventleakage of 100 mesh sand. A 2″ piece of vinyl tubing was placed onto thenipple of the insert and a clamp was fixed in place on the tubing priorto packing. The columns were gravity-packed by pouring approximately 25grams of the diluted microemulsions (e.g., the microemulsions describedin Example 1, and diluted with 2% KCl, e.g., to about 2 gpt, or about 1gpt) into the column followed by a slow, continuous addition of sand.After the last portion of sand had been added and was allowed to settle,the excess of brine was removed from the column so that the level ofliquid exactly matched the level of sand. Pore volume in the packedcolumn was calculated as the difference in mass of fluid prior to columnpacking and after the column had been packed. Three additional porevolumes of brine were passed through the column. After the last porevolume was passed, the level of brine was adjusted exactly to the levelof sand bed. Light condensate oil was then added on the top of sand bedto form the 5 cm oil column above the bed. Additional oil was placedinto a separatory funnel with a side arm open to the atmosphere. Oncethe setup was assembled, the clamp was released from the tubing, andtimer was started. Throughout the experiment the level of oil wasmonitored and kept constant at a 5 cm mark above the bed. Oil was addedfrom the separatory funnel as necessary, to ensure this constant levelof head in the column. Portions of effluent coming from the column werecollected into plastic beakers over measured time intervals. The amountof fluid was monitored. When both brine and oil were produced from thecolumn, they were separated with a syringe and weighed separately. Theexperiment was conducted for 2 hours at which time the steady-stateconditions were typically reached. The cumulative % or aqueous fluiddisplaced from the column over a 120 minute time period, and thesteady-state mass flow rate of oil at t=120 min through the column weredetermined.

Example 3

This example described a non-limiting experiment for determiningdisplacement of residual aqueous treatment fluid by formation gas. A 51cm long, 2.5 cm inner-diameter capped glass chromatography column wasfilled with approximately 410±20 g of 20/40 mesh Ottawa sand and thediluted microemulsions (e.g., the microemulsions described in Example 1,and diluted with 2% KCl, e.g., to about 2 gpt, or about 1 gpt). Toensure uniform packing, small amounts of proppant were interchanged withsmall volumes of liquid. Periodically the mixture in the column washomogenized with the help of an electrical hand massager, in order toremove possible air pockets. Sand and brine were added to completelyfill the column to the level of the upper cap. The exact amounts offluid and sand placed in the column were determined in each experiment.The column was oriented vertically and was connected at the bottom to anitrogen cylinder via a gas flow controller pre-set at a flow rate of 60cm³/min. The valve at the bottom was slowly opened and liquid exitingthe column at the top was collected into a tarred jar placed on abalance. Mass of collected fluid was recorded as a function of time by acomputer running a data logging software. The experiments were conducteduntil no more brine could be displaced from the column. The total % offluid recovered was then calculated.

Example 4

This example describes a method for determining the phase inversiontemperature of a solvent (e.g., a terpene). The methods are described inthe literature (e.g., see Strey, Microemulsion microstructure andinterfacial curvature. Colloid & Polymer Science, 1994. 272(8): p.1005-1019; Kahlweit et al., Phase Behavior of Ternary Systems of theType H₂O-Oil-Nonionic Amphiphile (Microemulsions). Angewandte ChemieInternational Edition in English, 1985. 24(8): p. 654-668.). As will beknown in the art, the PIT measured for a given oil or solvent depends onthe surfactant and aqueous phase in which it is measured. In thisexample, a 1:1 mixture of terpene solvent and de-ionized water wascombined with varying amounts of a 1:1 blend of Neodol 25-7 and IPA andthe upper and lower temperature boundaries of the one-phasemicroemulsion region were determined. A phase diagram such as this,plotting temperature against surfactant concentration at a constantoil-to-water ratio is often called a “fish” diagram or a Kahlweit plot.The phase inversion temperature was determined as the point on the“fish-tail” at which the temperature range of one-phase microemulsioncloses to a vertex. In this example, the temperature at the vertex wasselected as the PIT. An exemplary fish diagram indicating the PIT isshown in FIG. 1. For the terpene solvents used in this example, the PITvalues which were measured using this above-described procedure areshown in Table 1. Those terpenes containing alcohol groups (linalool,geraniol, nopol, α-terpineol and menthol), gave PIT values between −4°C. and 16° C. Eucalyptol, containing an ether-oxygen, and menthone,containing a carbonyl oxygen, gave somewhat higher values near 30° C.d-limonene gave 109.4° F., while other non-oxygen containing terpenesgave values between 48-58° C.

Example 5

A series of laboratory tests similar to as described in Example 1 wereconducted to characterize the effectiveness of a series ofmicroemulsions incorporating a range of terpenes. The phase inversiontemperatures of the terpenes were determined as described in Example 4.

Table 5 shows results for displacement of residual aqueous treatmentfluid by oil and gas for formulations (e.g., using the experimentalprocedures outlined in Examples 2 and 3) using dilutions of themicroemulsions comprising 46 parts of 1:1 Neodol 25-7, 27 partsdeionized water, and 27 parts terpene solvent). The dilutions wereprepared of each microemulsion in 2% KCl, at 2 gpt. The table shows thatthe terpene solvents with PIT values higher than 109.4° F. all giveapproximately 90% recovery, while those below 109.4° F. givesignificantly lower recovery. Table 5 also shows displacement by gasresults for the dilutions that demonstrates that terpene solvents withPIT values higher than 109.4° F. give approximately 40% recovery, whilethose with PIT values below 109.4° F. give significantly higherrecovery.

TABLE 5 PIT values for various terpene solvents (e.g., measured at 1:1water-oil). Displacement results for 2 gpt dilution of microemulsionscomprising 46:27:27 surfactant:water:terpene + isopropanol formulations.Displacement Phase Inversion of brine (%) Maximum Temperature by crudeoil displacementof Terpene ° F. (° C.) at 60 minutes brine (%) by gaslinalool 24.8 (−4) — 81.9 geraniol 31.1 (−0.5) 69.3 67.8 nopol 36.5(2.5) 80.3 58.8 α-terpineol 40.3 (4.6) 80 92.9 menthol 60.8 (16) 49.7 —eucalyptol 87.8 (31) — 54.6 menthone 89.6 (32) 79.4 — d-limonene 109.4(43) 89.3 45.6 terpinolene 118.4 (48) 90.5 41.8 β-occimene 120.2 (49)90.2 44.2 γ-terpinene 120.2 (49) 89 32.2 α-pinene 134.6 (57) 89.9 38.7citronellene 136.4 (58) 88.2 40.5

The results shown in Table 6 demonstrate that at a 1:1 ratio of terpeneto water, and 46 weight % surfactant-IPA, the high PIT α-pineneperformed better on oil displacement and much poorer on gas displacementthan the low PIT α-terpineol. As the terpene-to-water ratio decreasesfrom 27-27 to 21-33 to 11-43, the difference in oil displacementperformance decreased, then increased again at the lower level. Highersurfactant levels did not substantially increase or decrease thedisplacement (which may suggest that the microemulsion is performingdifferently than a surfactant package lacking the terpene solvent). Thedisplacement by gas was better for the low PIT α-terpineol than for thehigh PIT α-pinene.

TABLE 6 Oil and Gas displacement results for α-pinene and α-terpineol asa function of surfactant concentration and solvent-to-water ratio.Displacement of brine (%) Maximum Formulation by crude oildisplacementof T/S/W* Terpene at 60 minutes brine (%) by gas 27-46-27α-terpineol 80 92.9 27-46-27 α-pinene 89.9 38.7 21-46-33 α-terpineol 8883 21-46-33 α-pinene 87 46 11-46-43 α-terpineol 88.5 80 11-46-43α-pinene 96 47 15-56-28 α-terpineol 87.8 85 15-56-28 α-pinene 88.6 52*T/S/W stands for terpene weight %/1:1 surfactant-IPA weight %/deionizedwater wt %

It will be evident to one skilled in the art that the present disclosureis not limited to the foregoing illustrative examples, and that it canbe embodied in other specific forms without departing from the essentialattributes thereof. It is therefore desired that the examples beconsidered in all respects as illustrative and not restrictive,reference being made to the appended claims, rather than to theforegoing examples, and all changes which come within the meaning andrange of equivalency of the claims are therefore intended to be embracedtherein.

While several embodiments of the present invention have been describedand illustrated herein, those of ordinary skill in the art will readilyenvision a variety of other means and/or structures for performing thefunctions and/or obtaining the results and/or one or more of theadvantages described herein, and each of such variations and/ormodifications is deemed to be within the scope of the present invention.More generally, those skilled in the art will readily appreciate thatall parameters, dimensions, materials, and configurations describedherein are meant to be exemplary and that the actual parameters,dimensions, materials, and/or configurations will depend upon thespecific application or applications for which the teachings of thepresent invention is/are used. Those skilled in the art will recognize,or be able to ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described and claimed. Thepresent invention is directed to each individual feature, system,article, material, kit, and/or method described herein. In addition, anycombination of two or more such features, systems, articles, materials,kits, and/or methods, if such features, systems, articles, materials,kits, and/or methods are not mutually inconsistent, is included withinthe scope of the present invention.

The indefinite articles “a” and “an,” as used herein in thespecification and in the claims, unless clearly indicated to thecontrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in theclaims, should be understood to mean “either or both” of the elements soconjoined, i.e., elements that are conjunctively present in some casesand disjunctively present in other cases. Other elements may optionallybe present other than the elements specifically identified by the“and/or” clause, whether related or unrelated to those elementsspecifically identified unless clearly indicated to the contrary. Thus,as a non-limiting example, a reference to “A and/or B,” when used inconjunction with open-ended language such as “comprising” can refer, inone embodiment, to A without B (optionally including elements other thanB); in another embodiment, to B without A (optionally including elementsother than A); in yet another embodiment, to both A and B (optionallyincluding other elements); etc.

As used herein in the specification and in the claims, “or” should beunderstood to have the same meaning as “and/or” as defined above. Forexample, when separating items in a list, “or” or “and/or” shall beinterpreted as being inclusive, i.e., the inclusion of at least one, butalso including more than one, of a number or list of elements, and,optionally, additional unlisted items. Only terms clearly indicated tothe contrary, such as “only one of” or “exactly one of,” or, when usedin the claims, “consisting of,” will refer to the inclusion of exactlyone element or a list of elements. In general, the term “or” as usedherein shall only be interpreted as indicating exclusive alternatives(i.e. “one or the other but not both”) when preceded by terms ofexclusivity, such as “either,” “one of,” “only one of,” or “exactly oneof.” “Consisting essentially of,” when used in the claims, shall haveits ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “atleast one,” in reference to a list of one or more elements, should beunderstood to mean at least one element selected from any one or more ofthe elements in the list of elements, but not necessarily including atleast one of each and every element specifically listed within the listof elements and not excluding any combinations of elements in the listof elements. This definition also allows that elements may optionally bepresent other than the elements specifically identified within the listof elements to which the phrase “at least one” refers, whether relatedor unrelated to those elements specifically identified. Thus, as anon-limiting example, “at least one of A and B” (or, equivalently, “atleast one of A or B,” or, equivalently “at least one of A and/or B”) canrefer, in one embodiment, to at least one, optionally including morethan one, A, with no B present (and optionally including elements otherthan B); in another embodiment, to at least one, optionally includingmore than one, B, with no A present (and optionally including elementsother than A); in yet another embodiment, to at least one, optionallyincluding more than one, A, and at least one, optionally including morethan one, B (and optionally including other elements); etc.

In the claims, as well as in the specification above, all transitionalphrases such as “comprising,” “including,” “carrying,” “having,”“containing,” “involving,” “holding,” and the like are to be understoodto be open-ended, i.e., to mean including but not limited to. Only thetransitional phrases “consisting of” and “consisting essentially of”shall be closed or semi-closed transitional phrases, respectively, asset forth in the United States Patent Office Manual of Patent ExaminingProcedures, Section 2111.03.

What is claimed is:
 1. A method of treating a gas well having a wellbore, comprising: injecting a microemulsion into the wellbore to increase formation gas production by the well, wherein the microemulsion comprises water, a first type of solvent, and a first type of surfactant, wherein the microemulsion includes a continuous water phase, wherein the first type of solvent is selected from the group consisting of unsubstituted cyclic or acyclic, branched or unbranched alkanes having 6-12 carbon atoms, unsubstituted acyclic branched or unbranched alkenes having one or two double bonds and 6-12 carbon atoms, cyclic or acyclic, branched or unbranched alkanes having 9-12 carbon atoms and substituted with only an —OH group, branched or unbranched dialkylether compounds having the formula C_(n)H_(2n+1)OC_(m)H_(2m+1), wherein n+m is between 6 and 16, and aromatic solvents having a boiling point between about 300-400° F.
 2. The method of claim 1, wherein the ratio of water to solvent is between about 15:1 and 1:10.
 3. The method of claim 1, wherein the microemulsion further comprises a second type of solvent.
 4. The method of claim 1, wherein the microemulsion is diluted with an aqueous fluid prior to injection into the wellbore.
 5. The method of claim 4, wherein the microemulsion is diluted to between about 0.1 wt % and about 2 wt % versus the total aqueous fluid.
 6. The method of claim 4, wherein the microemulsion is diluted with an aqueous fluid selected from the group consisting of water, brine and a well-treatment fluid.
 7. The method of claim 6, wherein the well-treatment fluid is selected from the group consisting of an acid, a fracturing fluid and slickwater.
 8. The method of claim 1, wherein the microemulsion further comprises a freezing point depression agent.
 9. The method of claim 8, wherein the microemulsion comprises a first type of freezing point depression agent and a second type of freezing point depression agent.
 10. The method of claim 8, wherein the freezing point depression agent is selected from the group consisting of an alkylene glycol, an alcohol, a combination of choline chloride and urea, and a salt.
 11. The method of claim 8, wherein the freezing point depression agent is present in an amount between about 5 wt % and about 40 wt % versus the total microemulsion.
 12. The method of claim 1, wherein the surfactant is an alkyl polyglycol or alkyl polyglucoside nonionic surfactant.
 13. The method of claim 1, wherein the microemulsion further comprises a second type of surfactant.
 14. The method of claim 13, wherein the first type of surfactant is an alkyl polyglycol or alkyl polyglucoside nonionic surfactant and the second type of surfactant is an anionic, cationic, or zwitterionic surfactant.
 15. The method of claim 1, wherein the first type of surfactant is present in an amount between about 15 wt % and 55 wt % versus the total microemulsion. 